1. Field of the Invention
The present invention relates to a wet friction material used in a friction engaging device such as a clutch or a brake employed within oil in an automatic transmission of, for example, a vehicle.
2. Related Background Art
A wet friction material has been used in an automatic transmission of an automatic vehicle and the like. The automatic transmission of the automatic vehicle involves a multi-plate clutch in which a plurality of friction plates each formed by sticking wet friction materials to surfaces of a metallic substrate (core plate) and a plurality of separator plates as friction counterpart materials each formed by a single plate such as a metallic plate are alternately arranged and is designed so that a driving force is transmitted by abutting these plates against each other or interrupting by releasing these plates from each other within ATF (automatic transmission fluid) used as a lubricating oil.
As a wet friction material used in the oil in this way, a wet friction material of paper type called as “paper friction material” is generally used. This wet friction material is generally formed by paper-making fiber base materials such as natural pulp fibers, organic synthetic fibers or inorganic fibers with filler such as diatom earth or a cashew resin and a friction regulator in a wet manner and, then, by impregnating a binder containing a thermosetting resin and, subsequently, by curing with heat. As a resin binder used on this occasion, a phenol resin having excellent heat resistance, high mechanical strength and a good compression fatigue property has conventionally been mainly used.
By the way, in a recent industrial field regarding automobiles, light weight and high efficiency of various parts have been sought in order to save energy and to achieve light weight. On the other hand, there is a tendency for seeking high rotation and high output of an automobile engine. Also in an automatic transmission, in order to cope with the compactness and light weight of the friction engaging device and the high rotation and high output of the automobile engine, regarding the wet friction material, enhancement of the friction coefficient and further improvements in heat-resistance and endurance have been sought.
In order to resolve these problems, a technique for compounding high hard filler such as alumina to the paper base material has been investigated. However, in this technique, although the friction coefficient is great in an initial engaging condition, since an attacking force against the friction counterpart material is strong, as the usage of the friction material is advanced, the friction coefficient is gradually decreased. Thus, this technique is not satisfactory. Further, in the past, resins other than the phenol resin used as the binder, for example, a silicone resin has also been investigated. The silicone resin has excellent heat resistance and endurance based on siloxane bonding of main structure and does not generate a burned area or peeling-off on a friction material under a high temperature/high load condition and has excellent long term stability of friction coefficient. Further, since the silicone resin is soft, a contact area thereof with the friction counter material at the time of abutting is increased and, then, the friction coefficient is increased; however, a compression fatigue property thereof is bad and a displacement amount of an entire thickness of the friction material becomes great and a problem of a long-term stability of friction property remains unsolved. Thus, a satisfactory friction material could not be obtained. Further, a μ-V (friction coefficient versus velocity) property may have negative gradient, with the result that, when the friction material is used in a clutch, a transmission shock may occur.
That is to say, regarding the wet friction material, there is a need for seeking further improvement in three factors, that is, the high friction coefficient, the excellent compression fatigue property and the positive gradient of μ-V property.
Aiming for solving these problems, for example, in Japanese Patent Unexamined Publication No. JP-A-2002-363542, a resin composition for wet friction material to be obtained by mixing a resol-type phenol resin and a hydrolyzed solution of alkoxysilane or a lower condensate thereof is disclosed. Compared with an ordinary phenol resin, the friction coefficient or heat resistance has been improved. However, the compression fatigue property, particularly, the displacement amount of the entire thickness of the friction material under a high pressure becomes great, compared with the ordinary phenol resin. Thus, it can not be said that the resin composition is satisfactory.